Table 1.

Quality ratings of selected papers by study design

Author	YP	RN	Criterion of high quality	Cohort studies			Case-control studies
				Selected [A]	High quality [B]	[B]/[A] (%)	Selected [C]	High quality [D]	[D]/[C] (%)
Yang et al.	2011	[16]	7+/10	3	3	100	8	2	25
Wu et al.	2013	[17]	7+/10	11	10	91	24	11	46
Wu et al.	2013	[18]	7+/10	6	6	100	16	5	31
Zhu et al.	2013	[19]	7+/10	12	10	83	30	13	43
Choi et al.	2013	[20]	7+/9	4	4	100	23	5	22
Liu et al.	2014	[21]	8+/9	2	2	100	31	4	13
Wang et al.	2014	[22]	7+/9	17	14	82	0	0	-
Yang et al.	2014	[23]	7+/9	10	9	90	9	2	22
Song et al.	2014	[24]	7+/9	4	4	100	14	8	57
Xin et al.	2015	[25]	7+/9	5	4	80	11	3	27
Wang et al.	2015	[26]	7+/9	6	6	100	13	7	54
Wu et al.	2015	[27]	7+/9	1	1	100	21	10	48
Li et al.	2015	[28]	7+/9	4	4	100	5	1	20
Hu et al.	2015	[29]	7+/9	4	4	100	4	1	25
Total				89	81	(91)	209	72	(34)

YP, year of publication; RN, reference number.